1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to protective and monitoring arrangements for elevator systems which monitor the landing of an elevator car at a target floor.
2. Description of the Prior Art
An elevator car should consistently and reliably make smooth, accurate stops at the floors of the building it is serving, within predetermined velocity and deceleration ranges, and with appropriate operation of the car and hatch doors. Instead of waiting for the elevator car to stop level with a target floor before initiating the opening of the car and hatch doors, it is common to pre-open the doors in order to reduce floor-to-floor time and thus improve efficiency and service in a building. Pre-opening of the elevator doors is conventionally initiated when the elevator car is a predetermined distance from the floor, such as a distance in the range between two and ten inches, if the car speed at this point is below a predetermined magnitude, such as 150 FPM. A predetermined period of time after the doors are to open, such as 1.5 seconds, the elevator car must be within a predetermined distance from floor level, such as two inches, and traveling less than a predetermined speed, such as less than 30 FPM. If these requirements are not met, certain modifications to the normal operation of the elevator system may be instituted, such as by limiting the car speed and/or stopping the car and preventing it from being restarted until checked by operating personnel, as disclosed in U.S. Pat. Nos. 3,741,348 and 3,802,274, which are assigned to the same assignee as the present application. Additional monitoring functions provide back-up protection, such as monitors which check the armature voltage, and rate of change of armature voltage, of the drive motor, and, as disclosed in our U.S. Pat. No. 4,085,823, a monitor which constantly compares actual car velocity with the expected car velocity based upon the command or speed pattern signal. This latter monitoring function is especially accurate because the difference signal is unlike the error developed in response to the difference between the command signal and the car velocity. The conventional error signal is necessarily large during certain portions of the acceleration and deceleration phases, and thus a detector which monitors the magnitude of the error signal would have to be set to a relatively large magnitude in order to avoid nuisance tripping. On the other hand, comparing actual and expected car speeds enables a monitor which monitors the difference signal to be set to a relatively small value, as these two signals will normally track very closely at all stages of a run, when the elevator system is operating normally. This monitor, however, is not designed to recognize a failure in the command signal.